1. Field of the Invention
The invention relates to excimer and molecular fluorine laser systems, and particularly including at least one optical component made of MgF2.
2. Description of the Related Art
Line-narrowed excimer lasers are applied in the field of photolithography for production of integrated circuits. Achromatic imaging optics for this wavelength region are difficult to produce. For this reason, line-narrowed excimer laser radiation is generated for use in photolithographic applications in order to prevent errors caused by chromatic aberrations. Exemplary bandwidths for different imaging systems are tabulated in Table 1 for the excimer laser wavelengths 248 nm (KrF laser), 193 nm (ArF laser), and for the molecular fluorine laser wavelength 157 nm (F2-laser).
TABLE 1imaging optics248 nm193 nm157 nmrefractive optics:0.4-0.6 pm0.3-0.6 pm0.1 pmcatadioptics20-100 pm10-40 pmapprox. 1 pm
Internal optical resonator components may be made of CaF2, particularly for the ArF laser emitting around 193 nm and the F2 laser emitting around 157 nm. This relates to resonator optics such as mirrors or prisms, as well as to the laser tube windows that are also a part of the beam path within the resonator. It is recognized in the present invention that when such excimer or molecular fluorine laser is operated at high power (e.g., greater than 40 Watts)/high repetition rate (e.g., 2-4 kHz or more), as is desired for increasing throughput, then the tube windows, which include bulk material typically without coatings, tend to crack more easily in the particular case wherein the bulk material comprises CaF2. It may be that very small residual absorption within the CaF2 material or on the surface of the CaF2 substrate causes a strong thermal gradient and stress that the CaF2 component window is unable to withstand, such that the window then cracks.
It is further recognized in the present invention, that if the bulk material of the windows comprises MgF2, which is also tetragonal and is a material desirable for production at high purity and in large volumes, then the windows would be more suitable for withstanding high intensity and average power. In experiments, the MgF2 windows were in fact more resistant to cracking or complete destruction than CaF2 windows. The combination of the thermal and mechanical properties (hardness) of the MgF2 material is recognized herein as being superior for this application than CaF2. Even if the overall purity of the MgF2 material to be used is not as mature as the CaF2, the MgF2 is still advantageously more durable than CaF2 in a high power resonator.
A difficulty with using MgF2 in narrowband lithography type excimer or molecular fluorine lasers, however, is that MgF2 may typically have a negative effect on the degree of polarization of the generated beam, while it is desired that these narrowband lasers have a high degree of linear polarization of the laser output beam, e.g., greater than 95%. This effect is due to MgF2 being strongly birefringent, in contrast to CaF2, which may typically degrade the polarization of the beam as it traverses the MgF2 bulk material, which results ultimately in an evidently lower output energy for narrow band laser operation. For this reason, MgF2 has been typically avoided and CaF2 is the material of choice for sub-200 nm optical components. It is desired to circumvent this problem and achieve the durability benefits of forming high power, sub-200 nm laser optical components from MgF2. It is recognized in the present invention, in this regard, that the negative effect on polarization of the birefringent MgF2 material occurs particularly when the optical axis of the MgF2 component is oriented at an arbitrary angle to the beam.